The discovery of superconductivity in the La-Ba-Cu oxide system by Bednorz and Muller precipitated worldwide activity that very soon resulted in the discovery of other classes of oxide superconductors, frequently collectively referred-to as "high T.sub.c " superconductors. Among the classes of high T.sub.c superconductors is the class of Bi-Sr-Ca-Cu oxide superconductors. See, for instance, H. Maeda et al., Japanese Journal of Applied Physics, Vol. 27 (1988), L209; and U.S. Pat. No. 4,880,771 (incorporated herein by reference).
One of the problems that so far have prevented significant commercial use of "bulk" high T.sub.c superconductors is the relatively low current-carrying ability of bulk samples of these materials. It will be appreciated that bulk samples consist of many superconductor grains or crystallites that are packed to form a relatively dense body. The current-carrying ability is generally expressed in terms of the critical current density J.sub.c, which is a function of temperature and magnetic field.
There are at least two problems that contribute to the observed low values of J.sub.c in conventional bulk samples of high T.sub.c (by "high T.sub.c " we mean generally T.sub.c .gtorsim.30K, preferably&gt;77K; by "T.sub.c " we mean the highest temperature at which the D.C. resistance is zero to within experimental limits) oxide superconductors. One of the two problems is the so-called "weak link" problem. This pertains to the relatively low value of current that can flow without resistance from one superconductor grain to an adjoining one. This current frequently is referred to as the "inter-grain" current. The other is the so-called "flux flow" problem. This pertains to the relatively low current that can flow essentially without resistance within a given superconductor grain, due to weak flux pinning. Low values of critical current density are, however, not an inherent property of high T.sub.c oxide superconductors since current densities of the order of 10.sup.6 A/cm.sup.2 have been observed in thin films of high T.sub.c superconductor material.
Significant progress towards solution of the weak link problem has already been made. See S. Jin et al., Applied Physics Letters, Vol. 52(24), pp. 2074-2076, 1988; S. Jin et al., Applied Physics Letters, Vol. 54(6), pp. 584-586, 1989; and U.S. patent application Ser. No. 126,083, all incorporated herein by reference. The progress resulted from the discovery of so-called "melt-textured growth"(MTG), a processing technique that comprises (complete or partial) melting and oriented re-solidification of the superconductor material, resulting in highly textured material that can sustain significantly higher current densities than conventionally prepared bulk high T.sub.c superconductor material. Progress has also been made towards overcoming the flux flow problem. See, for instance, R. B. van Dover et al., Nature, Vol. 342, pp. 55-57; and U.S. patent application Ser. No. 07/442,285, filed Nov. 28, 1989 for S. Jin et al.
It has been observed previously that Ag-clad wires of Bi-Sr-Ca-Cu oxide superconductors can have quite high J.sub.c. For instance, K. Heine et al. (Applied Physics Letters, Vol. 55 (23), pp. 2441-2443) report making short lengths of Ag-clad Bi.sub.2 Sr.sub.2 CaCu.sub.2 O.sub.8+.times. (2212) wires by the "powder in tube"(see U.S. Pat. No. 4,952,554) method, and observing J.sub.c of up to 5.5.times.10.sup.4 A/cm.sup.2 at 4.2K and zero magnetic field, and up to 1.5.times.10.sup.4 A/cm.sup.2 at 4.2K and 26T. See also J. Tenbrink et al., Cryogenics, Vol. 30, pp. 422-426.
J. Kase et al. (Japanese Journal of Applied Physics, Vol. 29(7), pp. L1096-L1099; incorporated hereby by reference) report making a B.sub.2 Sr.sub.2 CaCu.sub.2 -oxide/silver composite "tape" by a method that involves forming a Bi-Sr-Ca-Cu-oxide-containing green tape by means of a doctor-blade process, cutting a sample (e.g., 30 mm.times.3 mm.times.50 .mu.m) from the green tape and laying the sample onto Ag foil. Subsequently the composite was subjected to a heat treatment that comprised a partial melting of the oxide material on the Ag foil. When cooling the composite slowly from the partially melted state (890.degree. C.) to 870.degree. C., followed by a quench from that temperature, strong c-axis alignment of the oxide material was observed. "C-axis alignment" refers to alignment of the crystallographic c-axis of an oxide crystallite essentially perpendicular to the substrate (in this case Ag-foil) surface. J.sub.c as high as 1.4.times.10.sup.5 A/cm.sup.2 at 4.2K and 25T were reported. The magnetic field is believed to have been parallel to the plane of the foil. See also D. R. Dietderich et al., Japanese Journal of Applied Physics, Vol. 29(7), pp. L1100-L1103, also incorporated herein by reference.
R. D. Ray et al. (Applied Physics Letters, Vol. 57(27), pp. 2948-2950; incorporated herein by reference) report forming Ag-clad Bi-Sr-Ca-Cu-oxide wires by the powder-in-tube method, followed by rolling the pulled-down powder-filled composite into ribbon shape, with thickness of 100 .mu.m. After melt processing and grinding off one side of the Ag cladding, X-ray analysis of the thus exposed oxide core showed pronounced c-axis alignment of the oxide crystallites.
Although substantial progress regarding increased J.sub.c has already been made, further increase would be desirable. A significant shortcoming however is the current lack of techniques capable of producing long lengths of high T.sub.c superconductors. For instance, Superconductor Week, Vol. 5(1), Jan. 7, 1991, on page 1 reports as follows: ". . . (Dr. Allen) Hermann agreed with (Dr. Douglas) Finnemore that increasing the length of the conductors is still a problem to some extent. However, he noted that researchers are now making meters of silver-sheathed wire." It will be readily appreciated that it will be necessary to make kilometer, not meter, lengths of conductors. A further shortcoming is the current lack of manufacturing techniques that could be adapted for continuous processing of long (e.g.,&gt;100 m) lengths of high T.sub.c superconductors.
For instance, using the powder-in-tube technique to produce long lengths of wire requires large-cross-section reduction of the starting "billet". Exemplarily, to obtain a 1 km length of wire from a 1 m long starting tube, a 1000:1 reduction in cross section is required. This will frequently be difficult to accomplish with a composite body such as a powder-filled metal tube. On the other hand, although the doctor-blade technique as used by Kase et al. (op. cit) could in principle be used to form long ribbons, the Kase et al. technique typically does not make efficient use of the superconductor material because it is frequently difficult to form thin (e.g.,&lt;10 .mu.m) layers by the doctor blade method, and because the interface-induced texture of the oxide material typically is confined to a few micron thickness adjacent to the Ag/oxide interface. Texture (more particularly, c-axis alignment) is required in order to achieve the desired high J.sub.c.
Furthermore, currently practiced techniques for making thin (e.g., &lt;1 .mu.m thick) superconductor layers typically involve such relatively expensive and complex techniques as sputtering, evaporation, or laser ablation. It is typically difficult to produce relatively large areas (e.g.,&gt;100 cm.sup.2) of high quality thin superconductor film by prior art techniques.
In view of the importance of the ability to produce long lengths of high J.sub.c superconductors, a method that can efficiently produce high T.sub.c superconductor ribbon of essentially unlimited length and high J.sub.c, and that can be readily embodied in a continuous process, would be of great significance. This application discloses such a method. It also discloses a method that can readily produce relatively large areas of thin superconductor sheet.